Contact the seller and ask for a firm shipping quote on the specific books you want. You have to be specific, because both USPS and Canada Post use dimensional standards for shipping rates.
If the seller is in the USA, also ask if he uses the Global Shipping Program and if he would make a private auction for you of the titles you want. Explain that the GSP does not seem to allow combined and discounted shipping,which combined with prepaid Canadian duty and sales taxes, would make the purchase unaffordable.
Be prepared to walk away if the seller is unwilling.
Politely- Captain Malcolm Reynolds.